This invention relates to a vacuum booster applicable to a brake unit of automotive vehicles and the like, for assisting an operator to operate a brake pedal, and more particularly to a vacuum booster of the type in which a valve body of a vacuum valve and a valve body of an atmospheric air valve are separately formed.
In general, a vacuum booster has a casing, a servo piston located within the casing, an input member disposed in a rearward position of the servo piston, and an output member disposed in a forward position of the servo piston. The servo piston is adapted to boost an operating force applied to the input member and exerts the same to output member. The interior of the casing is divided into a vacuum chamber on the forward side and a pressure-variable chamber on the rearward side by the servo piston. The boosting action is caused by a force corresponding to a pressure difference between those two chambers.
For the purpose of obtaining such a boosting action as just mentioned, the vacuum booster includes, as a control valve unit, a vacuum valve for allowing and cutting off a communication between the vacuum chamber and the pressure-variable chamber and an atmospheric air valve for cutting off and allowing a communication between the pressure-variable chamber and the atmospheric air. The valve bodies of those two valves are customarily integrally provided; for example, the vacuum valve body and the atmospheric air valve body are concentrically arranged on an outer periphery portion and an inner periphery portion of the same valve body (see, for example, U.S. Pat. No. 3,559,406 and Japanese Utility Model Publication No. 21744/92).
On the other hand, there is known an idea in which a valve body of a vacuum valve and a valve body of an atmospheric air valve are separately provided, in other words, the valve bodies are formed of separate members with each other, so that the valve bodies can be supported in a more stable manner and the opening and closing operation of the valves can more reliably be performed. The technique disclosed by Japanese Patent Publication No. 50695/82 is based on this idea. According to this idea, a valve body, which is supported by an inner periphery of an internal bore in a hub portion of a servo piston, serves as an atmospheric air valve, the valve body can sit on and leave away from an atmospheric air valve seat formed on a rear end of a valve plunger, a valve body slidably installed on an outer periphery of a valve plunger serves as a vacuum valve body so that they can sit on and leave away from a vacuum valve seat formed on an internal bore side of the hub portion.
The technique disclosed by the above Japanese Patent Publication No. 50695/82 indeed has the advantage that the valves can be opened and closed in a reliable manner owing to its structural feature of the valve bodies being separately provided but it also has the following shortcomings.
1 The above-mentioned technique employs an arrangement in which a stopper ring is provided on an outer periphery of the valve plunger as a means for allowing a communication (opening) of the vacuum valve when in an inoperable position so that the position of the vacuum valve body is restricted by the stopper ring, and in addition, a stopper member integral with the vacuum valve body is utilized as a means for causing the atmospheric air valve body to leave away from the atmospheric air valve seat when in an operating state. Accordingly, it becomes necessary that a clearance is provided between the stopper member and the atmospheric air valve body in order to prevent a possible inadvertent opening of the atmospheric air valve when in an inoperative position. This clearance tends to increase invalid stroke when in an operating state.
2 Since the vacuum valve body slidingly moves on the outer periphery of the valve plunger through a seal portion, a sliding resistance acts constantly thereon to thereby produce an output loss.
3 Since the vacuum valve body is mounted on the outer periphery of the valve plunger, the atmospheric air valve cannot be enlarged in diameter. Accordingly, a passage for allowing the atmospheric air to flow therein cannot be formed large in size and therefore, responsibility cannot be enhanced when in an operating state.